This application claims priority to Korean Patent Application No. 2004-58504 filed on Jul. 27, 2004 in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to image sensors, and in particular to an image sensor having adjusted spacing between micro-lenses and light receiving elements for improved photo sensitivity.
2. Description of the Related Art
An image sensor is generally implemented using semiconductor for converting light into electric signals. Recently, image sensors are classified into two types, i.e. a CCD (Charge Coupled Device) image sensor and a CMOS (Complementary Metal Oxide Semiconductor) image sensor.
The CCD image sensor accumulates electric charges generated in proportion to an intensity of light in each pixel. Such electric charge is converted into a voltage that is buffered and provided to an external device.
The CMOS image sensor includes control circuits and signal processing circuits as peripheral circuits. The CMOS image sensor also includes MOS (metal oxide semiconductor) transistors in each pixel for generating an output voltage at each pixel. The CMOS image sensor has low power consumption and is able to operate using a single power supply. Additionally, the CMOS image sensor is easier to drive, and the manufacturing process for the CMOS image sensor is simpler than for the CCD image sensor. Thus recently, the CMOS image sensor has been widely used in mobile devices.
The photo sensitivity of an image sensor is desired to be maximized, such as by a light convergence technique. Generally, the image sensor includes light receiving elements and logic circuits for converting light to an electric signal. When the size of the light receiving elements is increased, the intensity of received light is improved, but a total size of the image sensor is disadvantageously increased. The light convergence technique improves photo sensitivity while maintaining a smaller size of the image sensor.
FIG. 1 shows a cross-sectional view of a pixel array of a first conventional CMOS (Complementary Metal Oxide Semiconductor) image sensor (CIS). Referring to FIG. 1, the first conventional CIS includes a semiconductor substrate 102, an element layer 104, a metal layer 106, a color filter array 108, a coating layer 109, a micro-lens array 110, and an objective lens 111.
The element layer 104 is formed on the semiconductor substrate 102. The element layer 104 includes a plurality of pixels 101 composed of photodiodes PD used as light receiving elements and MOS (Metal Oxide Semiconductor) transistors (not shown in FIG. 1). Insulating structures 105 are formed for separating the photodiodes PD. The metal layer 106 is formed over the element layer 104, and includes a plurality of metal wires formed in a multi-layered configuration.
The color filter array 108 is formed over the metal layer 106. The coating layer 109 is formed on the color filter array 108 for controlling a focal distance to the photodiodes PD. A respective micro-lens is formed on the coating layer 109 in the micro-lens array 110 for converging light onto each of the photodiodes PD.
Each micro-lens has a predetermined refraction angle based on a size of a unit pixel 101 and a height or a disposition of the metal layer 106. Each micro-lens has a larger size than the corresponding photodiode PD below. The refraction angle for a micro-lens is determined by a thickness and a curvature radius of the micro-lens.
The objective lens 111 formed over the micro-lens array 110 condenses light onto the micro-lens array 110, and is supported by a CMOS image sensor module. The objective lens 111 has a predetermined refraction angle based on a size and a height of a CMOS image sensor module. The refraction angle of the objective lens 111 is determined by a thickness and a curvature radius of the objective lens 111.
FIG. 2 shows the first pixel array of FIG. 1 converging light onto the light receiving elements PD. Referring to FIG. 2, light is incident on the objective lens 111 (not shown in FIG. 2) that condenses such light onto the micro-lens array 110 according the refraction angle of the objective lens 111. Such light is further refracted by each of the micro-lenses 110 onto the photodiodes PD.
In such a CMOS image sensor structure of FIG. 2, a photodiode PD disposed in a center of the pixel array has maximum light intensity. Such a center of the pixel array is for a center pixel having an incident angle of light along an optical axis of the objective lens 111.
Meanwhile other photodiodes PD in peripheral portions of the pixel array, for example near ends of the pixel array, receive light of considerably lower intensity from variation of incident angles of light. As a result, such photodiodes PD in peripheral portions of the pixel array have undesirable photo sensitivity.
FIG. 3A shows a cross-sectional view of a pixel array of a second conventional CMOS image sensor. Referring to FIG. 3A, for maximizing intensity of light received at the photodiodes PD, centers of the micro-lenses 304a, 304b and 304c are disposed along the line segments Lna, LNb, and LNc between centers of corresponding photodiodes PD and a center of the objective lens (not shown in FIG. 3A). For the purpose of simplicity, FIG. 3A shows light passing through a center portion of the objective lens with maximum light intensity.
FIG. 3B shows the second pixel array of FIG. 3A with light converging on light receiving elements PD. Referring to FIG. 3B, the incident angle of light increases toward the ends of the pixel array from a center of the pixel array. The spacing between neighboring micro-lenses in FIG. 3B are narrower than those in FIG. 2. Therefore, each of the micro-lenses 304a and 304c in FIG. 3A is spaced from a respective end of the CMOS image sensor module 310 by a predetermined distance d0.
The predetermined distance d0 is proportional to the incident angle of light incident on the micro-lenses 304a and 304c and to a distance h between the micro-lens array 304 and the photodiodes PD. The CMOS image sensor of FIGS. 3A and 3B more efficiently prevent crosstalk and light shielding effect than the CMOS image sensor of FIGS. 1 and 2. However, when a pixel size is reduced or when the number of metal structures in the metal layer 306 is increased, the light shielding effect may predominate.
In that case, light travels very close to the metal structures of the metal layer 306 (as shown by the distances A0 and B0 in FIG. 3B). Such metal structures may block or scatter light away from the photodiodes PD resulting in reduced photo sensitivity.
FIG. 4 shows a cross-sectional view of a pixel array of a third conventional CMOS image sensor disclosed in Japanese patent number 10-32762. Referring to FIG. 4, centers of each of the photodiodes PD are formed at ends of line segments LNEa, LNEb, and LNEc drawn between a center of the objective lens (not shown) and centers of the micro-lenses 404a, 404b, and 404c, respectively. The CIS of FIG. 4 prevents crosstalk and shading effect. However, a size of the CIS of FIG. 4 may increase since the light receiving elements PD are shifted toward ends of the CIS module 410 in proportion to an incident angle of light and a distance between the micro-lenses 404a, 404b, and 404c and the photodiodes PD.